Soft pressure sensor using multi-material 3D-printed microchannel molds and method for making the sensor

ABSTRACT

The present invention relates to a flexible pressure sensor using a multi-material 3D-printed microchannel mold, and a method for manufacturing the same. An object of the present invention is to provide a flexible pressure sensor using a multi-material 3D-printed microchannel mold, the flexible pressure sensor being formed by using a conductive liquid and an elastomer, having a microchannel formed therein, and having improved flexibility, sensitivity, and stability in comparison to the related art. Another object of the present invention is to provide a method for manufacturing a flexible pressure sensor using a multi-material 3D-printed microchannel mold, in which the flexible pressure sensor is manufactured by using the microchannel mold including microbumps, the microchannel mold being multi-material 3D-printed by using a sacrificial material and a hard material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to Korean PatentApplication No. 10-2019-0050718 filed on Apr. 30, 2019 and Korean PatentApplication No. 10-2020-0050782 filed on Apr. 27, 2020 in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The following disclosure relates to a flexible pressure sensor using amulti-material 3D-printed microchannel mold, and a method formanufacturing the same, and more particularly, to a flexible pressuresensor with improved flexibility, sensitivity, and stability for use ina wearable device, and a method for manufacturing the same.

BACKGROUND

Health care and health monitoring mean measurement of vital signsnaturally generated from the body of a human, such as a pulse, a bloodpressure, and respiration, or monitoring of motions of a human such as amotion of a joint of a finger, a toe, or the like, a walk, and the like.With an increasing interest of people in health and welfare in modernsociety, investment and development of a technology that utilizes awearable device for smoothly performing the health care and healthmonitoring have been actively made for industrial purposes and academicpurposes.

A sensor capable of measuring various physical quantities such as aforce, a pressure, and a tensile strength is essential for measuring thevital signs and the body motion as described above. Further, there aremany requirements for the sensor as a wearable device to enable thehealth monitoring, the first of which is flexibility (elasticity). Theskin of a human is folded or stretched. That is, the skin of a human issubjected to various physical deformations such as stretching andfolding. Therefore, for the use as a wearable device, the sensor needsto have flexibility and elasticity as physical properties to bestretchable and foldable like the skin. In a case in which a sufficientelasticity is not provided, the device may be damaged or anuncomfortable feeling may be caused when a user wears the device, whichresults in deterioration in utilization efficiency.

Second, a sufficient sensitivity needs to be guaranteed, such that thesensor may sense the vital signs. The vital signs such as a pulse andrespiration are transferred through the skin. Therefore, a physicalquantity of the pressure is very small, and thus the measurement ispossible only with a high sensitivity with respect to a small pressure.Further, a sensor signal needs to be transmitted and received throughwireless communication to implement an actual use of the sensor as awearable device. In this case, when the sensitivity is low, the wirelesscommunication may not be performed smoothly, and thus the utilizationmay become difficult.

Finally, stability of the sensor signal needs to be guaranteed. Thewearable device is continuously used by the user in daily life.Therefore, a use frequency thereof is higher than that of other devices,and the wearable device is easily exposed to various external physicalenvironments. In order to implement the sensor with a long life span, aninitial value of the sensor signal needs to be stably maintained andstability for repetitive use needs to be ensured.

As such, studies and effort have been actively made to develop a sensorthat satisfies the requirements for utilization as a wearable device,that is, flexibility, sensitivity, and stability. For example, a methodof using a metal thin film by utilizing a structure capable oftolerating stretching, such as a serpentine pattern, or a method ofchanging resistance depending on stretching, pressurization, or the likeby mixing a conductive material in a polymer are used. However, in thecase of the first method, conductivity is high, but a tolerablestretching range is 30% or less, which is very limitative. In addition,peeling due to repetitive stretching and pressurization or the likeresults in a short life span. In the case of the second method,resistance is very high and viscoelasticity of the polymer itself isalso shown in an electric signal as it is. Therefore, recovery of thesignal is very slow, and strong hysteresis is shown.

To replace such methods, a study for manufacturing a sensor by using aconductive liquid that is suitably adapted even in a case of severedeformation and has a high conductivity has been actively conducted.Among the conductive liquids, a liquid metal has a high conductivity.However, the liquid metal also has a high surface tension, and thus amanufacturing method is complicated, and sensitivity is insufficient foruse as a resistive pressure sensor. Studies of utilizing the liquidmetal include Korean Patent Laid-Open Publication No. 10-2018-0102412(“soft sensor using 3D printing, method for manufacturing the same, andwearable device including the same”, published on Sep. 17, 2018,hereinafter, referred to as prior art). The prior art discloses aflexible sensor produced by injecting a conductive liquid metal on anelastic layer by using a syringe to form a microchannel. Particularly,in the prior art, a method of not using a mold is adopted to manufacturea flexible sensor with a smaller thickness, and the microchannel is3D-printed by using a syringe, instead of using a lithography processfor forming a microchannel as in the related art. In general, thelithography process requires various equipment and processing steps.Therefore, it is possible to greatly improve economical efficiency andproductivity by not using the lithography process.

However, in a case of manufacturing a flexible sensor by using themethod as the prior art, the thickness of the microchannel is determinedby a surface tension of the liquid metal. Therefore, it is not possibleto adjust the thickness of the microchannel as desired. In addition,there still is a limitation in sensitivity. That is, it is not possibleto achieve a sufficiently high sensitivity, which is a problem of thesensor using the liquid metal described above. Therefore, improvement isstill required.

RELATED ART DOCUMENT Patent Document

(Patent Document 1) Korean Patent Laid-Open Publication No.10-2018-0102412 (“soft sensor using 3D printing, method formanufacturing the same, and wearable device including the same”,published on Sep. 17, 2018)

SUMMARY

An embodiment of the present invention is directed to providing aflexible pressure sensor using a multi-material 3D-printed microchannelmold, the flexible pressure sensor being formed by using a conductiveliquid and an elastomer, having a microchannel formed therein, andhaving improved flexibility, sensitivity, and stability in comparison tothe related art. Another embodiment of the present invention is directedto providing a method for manufacturing a flexible pressure sensor usinga multi-material 3D-printed microchannel mold, in which the flexiblepressure sensor is manufactured by using the microchannel mold includingmicrobumps, the microchannel mold being multi-material 3D-printed usinga sacrificial material and a hard material.

In one general aspect, a flexible pressure sensor 100 using amulti-material 3D-printed microchannel mold includes: a flexible body110 formed of an elastomer and in which a microchannel 115 is formed; aconductive material 120 formed of a conductive liquid and filling themicrochannel 115; and a plurality of microbumps 130 formed of a hardmaterial and disposed to be in surface-contact with an upper surface ora lower surface of a part of the microchannel 115.

The flexible pressure sensor 100 may measure a pressure by using achange of a resistance value of the conductive material 120 that iscaused when the microchannel 115 filled with the conductive material 120is deformed due to an external pressure, and the microbumps 130 mayprevent the external pressure from being dispersed over the flexiblebody 110 to increase a degree of the deformation of the microchannel115.

Hereinafter, a direction in which the microchannel 115 extends will bereferred to as an extending direction, a direction parallel to athickness of the flexible pressure sensor 100 will be referred to as athickness direction, and a direction perpendicular to the extendingdirection and the thickness direction and parallel to a width of themicrochannel 115 will be referred to as a width direction.

The microchannel 115 may form a single route or multiple routes, andinclude a sensing portion S integrated in a predetermined shape at apredetermined position in the flexible body 110, and a reservoir Vdisposed to be spaced apart from the sensing portion S, positioned at adistal end of the route, having a cross-sectional area larger than thatof the sensing portion S, and to which a signal line 125 fortransmitting and receiving a signal to and from the outside isconnected. The microbumps 130 may be formed on the sensing portion S ofthe microchannel 115. The sensing portion S of the microchannel 115 maybe formed in a meandering channel shape. In the microchannel 115, awidth or thickness of the reservoir V may be larger than a width orthickness of the sensing portion S.

Sensitivity of the flexible pressure sensor 100 may be adjusted byadjusting a ratio (k_(t)=t/z) of a thickness (t) of the microbump 130 toa thickness (z) between a surface of the flexible pressure sensor 100that faces the microbump 130, and a surface of the microchannel 115 thatfaces the microbump 130, or by adjusting a ratio (k_(w)=d/w) of a width(d) of the microbump 130 to the width (w) of the microchannel 115.

The flexible pressure sensor 100 may have an embedded bump structure inwhich k_(t)<1 and k_(w)<1, an exposed bump structure in which k_(t)≥1,or an anchored bump structure in which k_(w)≥1.

As k_(t) is increased, the sensitivity of the flexible pressure sensor100 may be increased.

In a case in which 0<k_(w)<1, as k_(w) is increased, the sensitivity ofthe flexible pressure sensor 100 may be increased, and in a case inwhich k_(w)≥1, as k_(w) is increased, the sensitivity of the flexiblepressure sensor 100 may be decreased.

The microchannel 115 may include a connection portion T connecting thesensing portion S and the reservoir V, and the microbump 130 having theanchored bump structure may be formed in the connection portion T.

In another general aspect, a method for manufacturing the flexiblepressure sensor 100 using a multi-material 3D-printed microchannel moldincludes: a mold printing step of performing three-dimensional(3D)-printing of a micromold 140 having a 3D shape corresponding to ashape of the microchannel 115 by using a sacrificial material; a bumpprinting step of performing 3D-printing of the microbump 130 at apredetermined position on an upper surface of the micromold 140 by usinga hard material; a primary body forming step of primarily coating aflexible material that is a material of the flexible body 110 on asubstrate to form a flexible material layer; a mold disposing step ofdisposing a coupled body of the micromold 140 and the microbump 130 onan upper surface of the flexible material layer; a secondary bodyforming step of secondarily coating the flexible material on theflexible material layer and an upper surface of the coupled body andhardening the flexible material to form the flexible body 110 in whichthe coupled body is embedded; a channel forming step of forming themicrochannel 115 in the flexible body 110 by removing the micromold 140;and a manufacturing completion step of filling the microchannel 115 withthe conductive material 120 to complete manufacturing of the flexiblepressure sensor 100.

In another general aspect, a pulse measurement system includes: theflexible pressure sensor 100 described above that is attached to a wristof a user and measures a pulse; and three electrodes that are attachedto a body of the user, measure an electrocardiogram (ECG), and includeRef, In+, and In−.

The pulse measurement system may calculate a blood pressure of the userby using a pulse transit time which is a difference between a pulse peakpoint and an electrocardiogram peak point.

In another general aspect, a body pressure distribution measurementsystem includes: a plurality of flexible pressure sensors 100 describedabove that are distributed on clothes of a user and perform pressuremeasurement; and a monitoring unit that performs real-time monitoring ofa pressure applied to a body of the user by using a pressure measured bythe plurality of flexible pressure sensors 100.

The flexible pressure sensors 100 may be distributed at positionscorresponding to bony areas including shoulders, wing bones, elbows,knees, and a tailbone which are body parts that are likely to get abedsore.

Other features and aspects will be apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flexible pressure sensor according to the presentinvention.

FIG. 2 is an exploded perspective view of the flexible pressure sensoraccording to the present invention.

FIGS. 3A to 3C are photographs illustrating examples of the flexiblepressure sensor according to the present invention.

FIGS. 4A through 4F are views for describing an operational principle ofthe flexible pressure sensor according to the present invention.

FIGS. 5A through 5E illustrate comparison between the flexible pressuresensor according to the present invention and a flexible pressure sensoraccording to the related art.

FIGS. 6A to 6F illustrate a method for manufacturing a flexible pressuresensor according to the present invention.

FIGS. 7A to 7C are photographs illustrating manufacturing examples ofthe flexible pressure sensor according to the present invention.

FIG. 8 illustrates a result of a comparison experiment on the flexiblepressure sensor according to the present invention and a flexiblepressure sensor according to the related art.

FIGS. 9A to 9D are diagrams for describing finite element analysisaccording to a thickness of a microbump.

FIG. 10 illustrates a structure definition of the flexible pressuresensor according to the present invention.

FIGS. 11A to 11C illustrate various types of microbump structuresaccording to various values of k_(t) and k_(w).

FIGS. 12A through 12C illustrate an experiment result of a resistancechange with respect to a pressure according to various values of k_(t).

FIGS. 13A through 13C illustrate an experiment result of a resistancechange with respect to a pressure according to various values of k_(w).

FIG. 14 illustrates an example of the flexible pressure sensor accordingto the present invention to which an anchored bump structure is applied.

FIG. 15 illustrates an experiment result of a repetitive test.

FIGS. 16A to 16G and 17 are diagrams for describing a pulse measurementsystem using the flexible pressure sensor according to the presentinvention.

FIGS. 18A Through 18E; FIGS. 19A1, 19A2, 19B to 19 C and FIGS. 20Athrough 20D are diagrams for describing a body pressure distributionmeasurement system using the flexible pressure sensor according to thepresent invention.

DETAILED DESCRIPTION OF MAIN ELEMENTS

-   -   100: Flexible pressure sensor    -   110: Flexible body    -   115: Microchannel    -   120: Conductive material    -   125: Signal line    -   130: Microbump    -   140: Micromold

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, a flexible pressure sensor using a multi-material3D-printed microchannel mold according to the present invention with theabove-described configuration, and a method for manufacturing the samewill be described in detail with reference to the accompanying drawings.

[1] Flexible Pressure Sensor According to Present Invention

FIG. 1 illustrates an overall shape of a flexible pressure sensoraccording to the present invention, and FIG. 2 illustrates an explodedperspective view of the flexible pressure sensor according to thepresent invention. Further, FIGS. 3A to 3C are photographs illustratingexamples of the flexible pressure sensor according to the presentinvention. As illustrated in FIGS. 1 and 2 , a flexible pressure sensor100 according to the present invention may include a flexible body 110having a microchannel 115 formed therein, a conductive material 120, andmicrobumps 130. Hereinafter, the respective components will be describedin more detail.

The flexible body 110 refers to the whole body of the flexible pressuresensor 100 that is formed of a flexible material to be applicable to awearable device. As illustrated in FIG. 3A, the flexible body 110 isformed of an elastomer having a high flexibility to smoothly cope withvarious deformations such as stretching, folding, and twisting. Further,the flexible body 110 has the microchannel 115 formed therein and has aplanar shape for convenience of attachment to the body, clothes, or thelike.

The microchannel 115 may form a single route or multiple route asillustrated, and may include a sensing portion S and a reservoir V. FIG.3A is a photograph illustrating an example in which the microchannel 115forms a single route, and FIGS. 3B and 3C are photographs illustratingexamples in which the microchannel 115 forms multiple routes. Thesensing portion S is integrated in a predetermined shape at apredetermined position in the flexible body 110. For example, thesensing portion S may be formed in a meandering channel shape asillustrated in FIG. 3A. It is a matter of course that the presentinvention is not limited thereto. The sensing portion S may have anyshape suitable for needs and purposes, such as a spiral shape, anunfolded petal shape as illustrated in FIG. 3B, or a shape in which thesensing portion S is integrated over multiple sections as in FIG. 3C.The reservoir V is disposed to be spaced apart from the sensing portionS, positioned at a distal end of the route, having a cross-sectionalarea larger than that of the sensing portion S, and to which a signalline 125 for transmitting and receiving a signal to and from the outsideis connected. In FIGS. 1 and 2 , a case in which the sensing portion Sis formed on one side of the flexible pressure sensor 100 and thereservoir V is formed on the other side is illustrated. However, thepresent invention is not limited thereto. That is, various modificationsare possible. For example, the sensing portion S may be formed at acentral portion of the flexible pressure sensor 100 and the reservoir Vmay be formed on each of opposite end portions of the flexible pressuresensor 100. Alternatively, in a case of the example in which themicrochannel 115 forms multiple routes as illustrated in FIGS. 3B and3C, the sensing portion S may be formed at a central portion of theflexible pressure sensor 100 and the reservoir V may be formed on eachend portion of multiple routes.

The conductive material 120 is formed of a conductive liquid and fillsthe microchannel 115 to actually measure a pressure. The conductiveliquid may be a liquid metal as a specific example. The liquid metalrefers to a metal that is present in a liquid state at room temperature,such as mercury. The liquid metal has a high conductivity and thus maydrive a sensor with low power. In addition, the liquid metal is welladapted even in a case of various physical deformations, and thus anelectric property thereof is not lost. That is, a sensor manufactured ina form in which the conductive material 120 formed of the conductiveliquid fills the microchannel 115 in the flexible body 110 is highlysuitable as a wearable sensor, because a performance difference such asa change in basic resistance does not occur even in a case of variousphysical deformations such as stretching, folding, and twisting.

The microbump 130 is formed of a hard material. A plurality ofmicrobumps 130 are disposed so as to be in surface-contact with an uppersurface or a lower surface of a part of the microchannel 115. In thedrawings of the present specification, a case in which the microbumps130 are disposed on the upper surface of the microchannel 115 and anexternal pressure is applied from above is illustrated. However, it is amatter of course that the microbumps 130 may be disposed on the lowersurface of the microchannel 115 in a case in which the external pressureis applied from below. Here, the hard material refers to a material thatis solid. The hard material is not deformed even when an externalpressure is applied, unlike the flexible body 110 that is formed of aflexible material and deformed when the external pressure is applied.Further, the expression “micro” in the term “microbump” means that themicrobump 130 corresponds to the microchannel 115, and the expression“bump” (which will be described later in more detail) means anagglomerate formed on each of portions of the microchannel 115.

The flexible pressure sensor 100 is configured to measure a pressure byusing a change of a resistance value of the conductive material 120 thatis caused when the microchannel 115 filled with the conductive material120 is deformed due to an external pressure. A sensor only including theflexible body 110 and the conductive material 120 according to therelated art has been developed. However, in the sensor according to therelated art, resistance is not greatly changed by a small pressure,which is disadvantageous. However, according to the present invention,the microbumps 130 prevent the external pressure from being dispersedover the flexible body 110, and thus a degree of the deformation of themicrochannel 115 is increased. Therefore, it is possible to solve such aproblem of the related art and greatly improve the sensitivity of thesensor. In consideration of such use, it is preferable that themicrobumps 130 are formed on the sensing portion S of the microchannel115.

FIGS. 4A and 4B are views of different embodiments for describing anoperational principle of the flexible pressure sensor according to thepresent invention. Hereinafter, a direction in which the microchannel115 extends will be referred to as an extending direction, a directionparallel to a thickness of the flexible pressure sensor 100 will bereferred to as a thickness direction, and a direction perpendicular tothe extending direction and the thickness direction and parallel to awidth of the microchannel 115 will be referred to as a width direction.Here, views on the uppermost side of FIGS. 4A and 4B are views of a partof the microchannel 115 on which the microbump 130 are disposed, takenalong the extending direction, and views on FIGS. 4C and 4D arecross-sectional views, respectively, of the part of the microchannel 115taken along line A-A′ of FIGS. 4A and 4B, and views on FIGS. 4E and 4F,respectively, are cross-sectional views of the part of the microchannel115 taken along line B-B′ of FIGS. 4C and 4D.

FIG. 4A illustrates a state in which an external pressure is not appliedto the flexible pressure sensor 100. As illustrated in the view on theuppermost side of FIG. 4A, a thickness of the microchannel 115 at aregion corresponding to the sensing portion S is L, and a thickness ofthe microbump 130 is t. The thickness of the microchannel 115 is changeddepending on whether or not a pressure is applied, and a thickness ofthe microchannel 115 in an original state, that is, the thickness of themicrochannel 115 in a state in which no pressure is applied is L₀.Further, as illustrated in the views on the middle side and thelowermost side of FIG. 4A as shown in FIGS. 4C and 4E, respectively, awidth of the microchannel 115 at the region corresponding to the sensingportion S is w, and a width of the microbump 130 is d. Here, across-sectional area A of the microchannel 115 is obtained bymultiplying the width by the thickness (that is, wL). Further, althoughnot illustrated, an overall length of the microchannel 115 correspondingto the sensing portion S is 1.

In addition, as described above, the reservoir V is positioned at adistal end of a single route or each of multiple routes formed by themicrochannel 115, and has a cross-sectional area larger than that of thesensing portion S. That is, specifically, the width or thickness of thereservoir V may be larger than the width or thickness of the sensingportion S. FIGS. 4A and 4B illustrate an example in which both of thewidth and the thickness of the reservoir V are larger than the width andthe thickness of the sensing portion S as illustrated in the views onthe middle side and the lowermost side. FIGS. 1 and 2 illustrate anexample in which the reservoir V and the sensing portion S have the samethickness, but the width of the reservoir V is larger than the width ofthe sensing portion S.

FIG. 4B illustrates a state in which an external pressure is applied tothe flexible pressure sensor 100. Once an external pressure is appliedto the flexible pressure sensor 100, as illustrated in FIG. 4B, aportion to which the pressure is applied, that is, the regioncorresponding to the sensing portion S of the flexible pressure sensor100 is compressed and deformed, and naturally, the microchannel 115 inthe region corresponding to the sensing portion S is also compressed anddeformed. Therefore, as illustrated in the views on the middle side andthe lowermost side of FIG. 4B, as shown in FIGS. 4D and 4F, respectivelythe conductive material 120 in the microchannel 115 in the regioncorresponding to the sensing portion S flows to a portion to which nopressure is applied, that is, to a region corresponding to the reservoirV. Therefore, the region corresponding to the reservoir V is expandedand deformed when the region corresponding to the sensing portion S iscompressed and deformed. That is, since the microchannel 115 itselfforms a closed surface as a single route or multiple route, an overallvolume is not changed. However, as the region corresponding to thesensing portion is compressed and deformed, an amount of the conductivematerial 120 in the microchannel 115 in the region corresponding to thesensing portion S is definitely changed.

As such, in a case in which the pressure is removed in a state in whichthe pressure is applied to the region corresponding to the sensingportion S, the region corresponding to the sensing portion is compressedand deformed, and the region corresponding to the reservoir V isexpanded and deformed, the conductive material 120 formed of theconductive liquid concentrated in the reservoir V due to an internalpressure of the microchannel 115 appropriately returns to the sensingportion S, such that the flexible pressure sensor 100 is restored to anoriginal state as illustrated in FIG. 4A.

Resistance is determined based on a length and a cross-sectional area ofa resistor. As described above, the length 1 of the microchannel 115 maybe regarded as a value that is not changed, and the cross-sectional areaA of the microchannel 115 may be wL. That is, resistance R of the regioncorresponding to the sensing portion S may be calculated by thefollowing Equation.R=ρl/A=ρl/wL(R: resistance, ρ: resistivity constant of the material, l: length, A:cross-sectional area, w: width, and L: thickness)

Here, in a case in which the thickness of the microchannel 115 when nopressure is applied is L₀, a thickness of a portion of the microchannel115 where the microbump 130 is not positioned when a pressure is appliedis L₁, and a thickness of a portion of the microchannel 115 where themicrobump 130 is positioned when the pressure is applied is L₂, thefollowing relationship is established between these three thicknesses.L ₀ >L ₁ >L ₂

As such, in the flexible pressure sensor 100 according to the presentinvention, the thickness of the microchannel 115 is drastically changedto L₁ or L₂ when an external pressure is applied, and accordingly, theresistance change depending on the pressure change becomes more drastic.

FIGS. 5A to 5D illustrate comparison between the flexible pressuresensor 100 according to the present invention, that is, the flexiblepressure sensor 100 with the microbump 130 and a flexible pressuresensor without the microbump 130 according to the related art.

In a case in which the flexible pressure sensor 100 does not include themicrobump 130, that is, in a case of the sensor using the liquid metalaccording to the related art, an external energy is used to deform anelastomer and move a fluid. Therefore, the energy is consumed due toviscoelasticity of the elastomer itself, and thus an internalcross-sectional area is not greatly changed. As a result, a problem thatresistance is not greatly changed by a small pressure, that is, aproblem that sensitivity is low occurs.

However, according to the present invention, the microbump 130 formed ofthe hard material is in surface-contact with the microchannel 115 filledwith the conductive material 120 formed of the conductive liquid.Therefore, it is possible to prevent the external pressure from beingdispersed over the flexible body 110 formed of the elastomer as much aspossible, and the whole pressure may be used to decrease thecross-sectional area of the microchannel 115.

FIGS. 5A and 5C illustrates the sensor according to the related art,FIGS. 5B and 5D illustrates the flexible pressure sensor 100 accordingto the present invention, and FIG. 5E illustrates a change of theresistance R with respect to time t in each of the sensor according tothe related art and the flexible pressure sensor 100 according to thepresent invention. It is seen that the resistance change is more drasticin the flexible pressure sensor 100 with the microbump 130 according tothe present invention (w/bump), in comparison to the flexible pressuresensor without the microbump 130 according to the related art (w/o bump)as illustrated in FIG. 5E. That is, it may be appreciated that as theflexible pressure sensor 100 according of the present invention includesthe microbump 130, the sensitivity may be greatly improved as comparedwith the sensor according to the related art.

[2] Method for Manufacturing Flexible Pressure Sensor According toPresent Invention

FIGS. 6A to 6F schematically illustrate a method for manufacturing theflexible pressure sensor according to the present invention, and FIGS.7A to 7C are photographs illustrating manufacturing examples of theflexible pressure sensor according to the present invention. Steps ofthe method for manufacturing the flexible pressure sensor according tothe present invention will be described as follows with reference toFIGS. 6A to 7C.

FIG. 6A illustrates both of a mold printing step and a bump printingstep. As illustrated in FIG. 6A, in the mold printing step, a micromold140 having a three-dimensional shape corresponding to the shape of themicrochannel 115 is 3D-printed using the sacrificial material. Next, inthe bump printing step, the microbump 130 is 3D-printed at apredetermined position on an upper surface of the micromold 140 by usingthe hard material. The micromold 140 is to be removed later, and thus itis preferable that the micromold 140 is formed of an easy-to-removematerial such as a water-soluble material.

For example, a dual nozzle FDM 3D printing method may be used, in whicha water-soluble polyvinyl alcohol (PVA) filament may be printed from anozzle for manufacturing the micromold 140, and a polylactic acid (PLA)filament which is a material with high hardness may be printed from theother nozzle for manufacturing the microbump 130. The soluble filamentserves to form the shape of the microchannel into which the conductiveliquid is to be inserted, and the hard filament serves as the microbumpfor increasing the sensitivity of the sensor. For reference, FIG. 6Fillustrates examples of the materials used for each component.

FIG. 7A is photographs illustrating examples of a coupled body of themicromold 140 and the microbump 130 that are formed by performing themold printing step and the bump printing step, respectively, when viewedfrom above. As illustrated, the coupled body may be designed andmanufactured in various sizes and shapes for various needs such as acase of measuring a pressure of a large area such as a body part, or acase of measuring a small area, for example, measuring a pulse. FIG. 7Bis photographs illustrating cross sections of examples of the coupledbody of the micromold 140 and the microbump 130. It may be seen that itis possible to smoothly manufacture a very thin and elaborate object byusing 3D printing.

FIG. 6B illustrates both of a primary body forming step and a molddisposing step. As illustrated in FIG. 6B, in the primary body formingstep, a flexible material that is the material of the flexible body 110is primarily coated on a substrate to form a flexible material layer.Next, in the mold disposing step, the coupled body of the micromold 140and the microbump 130 is disposed on an upper surface of the flexiblematerial layer. In this step, it is preferable that the signal line 125is disposed at a portion corresponding to the reservoir V of themicromold 140.

FIG. 6C illustrates a secondary body forming step. In the secondary bodyforming step, the flexible material is secondarily coated on theflexible material layer and an upper surface of the coupled body andhardened to form the flexible body 110 in which the coupled body isembedded. As such, the coupled body may be smoothly embedded in theflexible body 110 by sequentially performing the primary body formingstep and the secondary body forming step. FIG. 7C is a photographillustrating an example of a step of manufacturing a flexible body inwhich multiple coupled body are embedded.

FIG. 6D illustrates a channel forming step, and in the channel formingstep, the micromold 140 is removed and the microchannel 115 is formed inthe flexible body 110. As described above, the micromold 140 is formedof an easy-to-remove material such as the PVA filament. Therefore, themicromold 140 may be easily removed by forming a small hole in one sideof the flexible body 110 in which the coupled body is embedded, andinjecting a solvent of the micromold 140 through the hole. Once themicromold 140 is removed as described above, a space in which themicromold 140 has been present becomes empty, and the empty space servesas the microchannel 115.

FIG. 6E illustrates a manufacturing completion step, and in themanufacturing completion step, the microchannel 115 is filled with theconductive material 120, thereby completing the manufacturing of theflexible pressure sensor 100. More specifically, the conductive material120 is injected through the hole formed for the removal of the micromold140 as described above and the hole is stopped, thereby completing themanufacturing of the sensor. As described above, the conductive material120 is most preferably a conductive liquid. For example, the conductivematerial 120 may be galinstan which is one of the liquid metals. Thegalinstan is an alloy of gallium, indium, and tin, is harmless to thehuman body, and has a high electric conductivity. Therefore, thegalinstan is significantly suitable for use in such a sensor utilizing aconductive liquid. Meanwhile, in a case in which multiple coupled bodiesare embedded in one elastomer layer as illustrated in FIG. 7C, a step ofappropriately determining a shape of the flexible body 110 byappropriately cutting the elastomer layer may be further performedlater.

[3] Result of Experiment on Flexible Pressure Sensor According toPresent Invention

FIG. 8 illustrates a result of a comparison experiment on the flexiblepressure sensor according to the present invention and the flexiblepressure sensor according to the related art. As a result of observing aresistance change with respect to a pressure, when a pressure of 50 kPais applied, the resistance increased by 0.5 times in the sensor (thesensor according to the related art) without the microbump, but theresistance increased by two times in the sensor (the sensor according tothe present invention) with the microbump as illustrated in FIG. 8 .Therefore, it can be appreciated that the sensitivity increased by fourtimes. As theoretically and briefly described above, the result of FIG.8 clearly shows that the sensitivity of the flexible pressure sensor 100according to the present invention is much higher than that of thesensor according to the related art.

FIGS. 9A to 9D are diagrams for describing finite element analysisaccording to the thickness of the microbump, which illustrate a resultof performing a finite element analysis simulation to more certainlyanalyze the above-described sensitivity improvement effect. FIG. 9Aillustrates a case in which no microbump is present, FIG. 9B illustratesa case in which the thickness t of the microbump is 200 μm, and FIG. 9Cillustrates a case in which the thickness t of the microbump is 400 μm.Further, cross-sectional area changes when a pressure is applied in therespective cases are illustrated. As shown in a table of FIG. 9D, it maybe appreciated that a decrease in cross-sectional area (an increase inresistance) was more significant in a case in which the microbump ispresent, than in a case in which no microbump is present. Further, thelarger the thickness of the microbump was, the more significant thedecrease in cross-sectional area (the increase in resistance) was. Asmay be inferred from such a simulation result, a larger thickness t ofthe microbump 130 is preferred to increase an influence of the microbump130. However, in a case in which the thickness t of the microbump 130 isexcessively large, the thickness of the flexible pressure sensor 100 maybecome unnecessarily large. In other words, it is preferable that thethickness t of the microbump 130 is appropriately determined to be thesame as, larger than, or smaller than the thickness L of themicrochannel 115, in consideration of both of the increase in resistanceand the thickness of the sensor.

As described above, the thickness t of the microbump 130, the width d ofthe microbump 130, the width w of the microchannel 115, and thethickness of the flexible pressure sensor 100 itself affect one another.Therefore, to more specifically define a relationship therebetween, FIG.10 illustrates a structure definition of the flexible pressure sensoraccording to the present invention, and an experiment was conducted.Signs of respective parts illustrated in FIG. 10 and a relationshiptherebetween are as follows.

t: thickness of microbump

d: width of microbump

L: thickness of microchannel

w: width of microchannel

z: thickness between microbump-side sensor surface and microbump-sidechannel surface

k_(t)=t/z

k_(w)=d/w

σ₀: pressure applied to microbump-side sensor surface

χ: ratio of pressure transferred to microchannel

FIGS. 11A to 11C illustrate various types of microbump structuresaccording to various values of k_(t) and k_(w).

FIG. 11A illustrates an embedded bump structure, in which the microbump130 is completely embedded in the flexible body 110 (k_(t)<1), and thewidth d of the microbump 130 is smaller than the width w of themicrochannel 115 (k_(w)<1). In this case, a pressure actuallytransferred to the microchannel is smaller than a pressure (σ₀) appliedto the microbump-side sensor surface (χ<1). Such a structure issignificantly stable, and thus may be widely used.

Meanwhile, FIG. 11B illustrates an exposed bump structure, in which themicrobump 130 is exposed from the flexible body 110 (k_(t)=1) orprotrudes from the flexible body 110 (k_(t)>1) so that an externalpressure is directly applied to the microbump 130 (k_(t)≥1). In thiscase, the external pressure applied to the microbump 130 is transferredto the microchannel 115 as it is, and thus the ratio of transferredpressure is 1 (χ=1). FIG. 11C illustrates an anchored bump structure, inwhich the width d of the microbump 130 is the same as or larger than thewidth w of the microchannel 115 (k_(w)≥1).

In the above description, the wide use is enabled by applying theembedded bump structure of FIG. 11A to the flexible pressure sensor 100.However, in a case in which there is a special need, an appropriatedesign change may be made for the need by appropriately mixing theexposed bump structure of FIG. 11B and the anchored bump structure ofFIG. 11C.

FIGS. 12 through 12C illustrate an experiment result of a resistancechange with respect to a pressure according to various values of k_(t),and it may be appreciated that as the value of k_(t) is increased, thesensitivity is drastically increased, as compared with a case in whichk_(t)=0 (that is, a case in which no microbump is present). For example,in a case in which the flexible pressure sensor 100 needs to measure avery weak signal like a pulse, or in a case in which the flexiblepressure sensor 100 needs to have a very small size, it is much moreadvantageous for the flexible pressure sensor 100 to have the exposedbump structure as illustrated in FIG. 11B.

FIGS. 13A through 13C illustrate an experiment result of a resistancechange with respect to a pressure according to various values of k_(w).In a case in which the value of k_(w) is smaller than 1 (that is, thewidth of the microbump<the width of the microchannel, k_(w)<1), as thevalue of k_(w) is increased, the sensitivity is increased (in the samecontext as the experiment result of FIG. 8 described above), as comparedwith a case in which k_(w)=0 (that is, a case in which no microbump ispresent). On the contrary, in a case in which the value of k_(w) islarger than 1 (that is, the width of the microbump>the width of themicrochannel, k_(w)>1), as the value of k_(w) is increased, thesensitivity is decreased.

As illustrated in FIG. 1 and the like, the flexible pressure sensor 100includes the sensing portion S where pressure sensing is performed, andthe reservoir V for transmission and reception of a signal to and fromthe outside, and the sensing portion S and the reservoir V areappropriately spaced apart from each other. Here, it is preferable thata portion (hereinafter, referred to as connection portion T) between thesensing portion S and the reservoir V has a structure in which a signalis not changed even when a pressure is applied (that is, the connectionportion T has a low sensitivity). In this case, it is advantageous forthe flexible pressure sensor 100 to have the anchored bump structure asillustrated in FIG. 11C. FIG. 14 illustrates an example of the flexiblepressure sensor according to the present invention to which the anchoredbump structure is applied. More specifically, FIG. 14 illustrates anexample in which the anchored bump structure as illustrated in FIG. 11Cis applied to the connection portion T between the sensing portion S andthe reservoir V. In this case, even when an unexpected external pressureis applied to the connection portion T, since the connection portion Thas a low sensitivity, it is possible to prevent an unnecessary noise ina process in which the pressure sensed at the sensing portion S istransferred to the reservoir V as much as possible.

FIG. 15 illustrates an experiment result of a repetitive test. That is,the repetitive test was performed to check whether or not the flexiblepressure sensor 100 according to the present invention manufactured asdescribed above shows a predetermined resistance change with repetitiveapplication of a pressure. As a result, it may be seen that the flexiblepressure sensor 100 showed the predetermined resistance change for 5000times or more with respect to the constant and repetitive application ofa pressure. That is, it may be seen from the result that the sensorusing the conductive liquid has a high signal stability. Such acharacteristic greatly affects the life span of the wearable device. Thehigher the signal stability, the longer the life span of the sensorbecomes.

[4] Utilization Examples of Flexible Pressure Sensor According toPresent Invention

There are various methods of utilizing the flexible pressure sensor 100according to the present invention manufactured as described above inhealth monitoring application. Hereinafter, a pulse measurement systemthat measures vital signs, and a body pressure distribution measurementsystem that measures a body motion will be described by way of example.

FIGS. 16A to 16G and FIG. 17 are diagrams for describing a pulsemeasurement system using the flexible pressure sensor according to thepresent invention.

A pulse is an index that most simply indicates information on a currenthealth condition of a user in a wearable device. First, referring toFIGS. 16A to 16G, the developed flexible pressure sensor may measure asmall pressure from a surface of the skin, the small pressure beingtransferred by constriction and dilation of blood vessels in the wrist(FIG. 16A). The intensity of the pulse transferred from the bloodvessels in the wrist varies depending on a position and each person.Therefore, the size of the sensor may be changed to a size most suitablefor measurement for each user. Basically, the sensor may have a pressuremeasurement region having a size 10×10 mm or 20×10 mm to perform thepulse measurement. Further, the pressure sensor is manufactured in aform of a bracelet that may surround the wrist, and the pulsemeasurement is performed.

In addition, an algorithm that may measure a blood pressure by usingelectrocardiogram (ECG) data (FIG. 16B). FIG. 16C is a pulse signalgraph with respect to time at the lowest point (green dot) in FIG. 16B,and FIG. 16D is a pulse signal graph with respect to time at the highestpoint (red dot) in FIG. 16B. It may be intuitively appreciated fromFIGS. 16C and 16D that a pulse rate is increased after exercise. Theblood pressure is an index indicating medically more meaningfulinformation as compared with the pulse. There have been various attemptsto simply estimate the blood pressure by using various methods. As oneof the attempts, various algorithms for estimating the blood pressure byusing data of a pulse transit time (PTT) have been developed, the PTTbeing a difference between a peak point in electrocardiogram and a peakpoint in pulse data. FIG. 16E is an experiment overview diagram forchecking a change of the PTT before and after exercise by measuring thepulse and the ECG at the same time. Here, Ref, In+, and In− indicatethree electrodes required for the ECG measurement. FIGS. 16E and 16Fillustrate results showing a decrease of the PTT before and after theexperiment, and it may be indirectly appreciated from the result thatthe blood pressure increased.

FIG. 17 illustrates a result of measuring a change of the pulsedepending on a degree of physical activity by using the flexiblepressure sensor according to the present invention. As clearlyillustrated in FIG. 17 , a change of the pulse signal is clearly shownin each of a normal state, an exercise state, and a rest state, and itmay be appreciated that the flexible pressure sensor according to thepresent invention sensitively measures the change.

In addition, as described above, it is preferable that the microbump 130included in the flexible pressure sensor 100 has the exposed bumpstructure to more sensitively measure a pulse which is a weak signal, inthe pulse measurement system.

FIGS. 18A to 18E, FIGS. 19A1, 19A2, 19B-19-C and FIGS. 20A through 20 Dare diagrams for describing a body pressure distribution measurementsystem using the flexible pressure sensor according to the presentinvention.

Measurement of body pressure distribution may be very useful as an indexfor estimating a posture of a patient lying on a bed or a body leaningon another object such as a chair. In particular, a stroke patient or apatient who is unable to move his/her body is highly likely to get asecondary disease such as a bedsore when lying on a bed for a long time,and thus there is a need to periodically change a posture of such apatient. In this case, it is possible to configure a system capable ofsimply notifying a carer or a nurse of an uncomfortable body part or afact that a certain pressure is applied to one body part for a certaintime, which may be noticed by no one other than the patient, by mainlymonitoring body pressure distribution around a body part that is likelyto get a bedsore.

Examples of the body part that is likely to get a bedsore include bonyareas such as shoulders, wing bones, elbows, knees, and a tailbone.Representative parts are selected and multiple pressure sensors areattached to clothes in array (FIG. 18A). The attached flexible pressuresensor is firmly attached to fibers of the clothes and thus is noteasily detached (FIG. 18B). A circuit is implemented using Arduino tocollect data transferred from the multiple pressure sensors (FIG. 18C),and a system in which real-time monitoring may be performed by apersonal computer (PC) through the circuit may be developed (FIG. 18D).

In an experiment actually performed using the developed system, a changeof a signal caused by a change from a state of lying on the back on abed (FIG. 19A1) to a state of lying on the side on the bed (FIG. 19A2)or a change from the state of lying on the side on the bed to the stateof lying on the back on the bed was checked. In a case illustrated inFIG. 19A1, the entire body is in contact with the bed, and thus apressure was transferred to the sensor as it is. However, in a case ofthe change to the state of FIG. 19A2, no pressure is applied to a sensorpositioned on the back of the body, and only a sensor positioned on theside of the body senses a pressure (FIG. 19B). On the contrary, in acase of the change from the state of FIG. 19A2 to the state of FIG.19A1, it may be seen that a signal value of the sensor positioned on theside of the body is decreased, and a signal value of the sensorpositioned on the back of the body is increased (FIG. 19C). As a result,it may be appreciated that the pressure sensor array system attached tothe clothes may be utilized to grasp and monitor the body motion.

FIGS. 20A through 20D illustrate an example in which the flexiblepressure sensor according to the present invention is attached to a heeland a heel pressure is monitored. In a case in which sensors areattached at intervals to a heel as illustrated in drawings of FIG. 20A,a larger pressure is applied to a sensor S #1 attached to the center ofthe heel when in a neutral state in which a user sits with legs straightout in a slight tension state, and a larger pressure is applied to asensor S #2 attached to the outer side of the heel when in a relaxedstate in which the user is in a completely relaxed state as shown inFIG. 20B. A drawing of FIG. 20C illustrates a pressure signal (convertedinto a voltage V) measured with respect to time in the neutral/relaxedstate, and FIG. 20D illustrates a pressure change depending on aposture, and it may be appreciated that the measurement of a heelpressure may be very smoothly performed.

According to the present invention, as the flexible pressure sensorincludes the conductive liquid that has a high conductivity and ishighly adaptive to various physical deformations, the elastomer that hasa high viscoelasticity, and the microbumps, it is possible to greatlyimprove flexibility, sensitivity, and stability as compared to therelated art. More specifically, since the flexible pressure sensoraccording to the related art is formed by simply injecting theconductive liquid into the microchannel formed in the elastomer, it isnot possible to obtain a sufficient sensitivity. However, according tothe present invention, the microbump is added on the microchannel, anexternal pressure is not dispersed due to the microbump, and may becompletely used to cause drastic deformation of the microchannel.Therefore, it is possible to drastically improve the sensitivity severaltimes as compared to the related art.

Further, according to the present invention, the flexible pressuresensor is manufactured by using the microchannel mold that ismulti-material 3D-printed by using the sacrificial material and the hardmaterial. As the microchannel mold is manufactured by using 3D printing,the manufacturing may be performed with low costs, in comparison tousing the lithography method as in the related art. Further, by adoptingthe method of using a mold, it is possible to fundamentally prevent aproblem of the related art that the thickness of the microchannel maynot be adjusted as desired, and it is possible to manufacture the sensorwith desired dimensions. In addition, by using the manufacturing methodusing a multi-material as described above, it is possible to easilyinsert the microbump for improving the sensitivity into the flexiblepressure sensor, and it is also possible to manufacture variousmulti-dimensional microchannels with various shapes through 3Dpatterning.

As such, according to the present invention, it is possible tomanufacture, in a much easier manner with a high degree of freedom, theflexible pressure sensor with greatly improved flexibility, sensitivity,and stability in comparison to the related art. Therefore, it ispossible to improve performance of a wearable device in which theflexible pressure sensor is used, and greatly improve utilizationefficiency.

The present invention is not limited to the abovementioned exemplaryembodiments, but may be variously applied. In addition, the presentinvention may be variously modified by those skilled in the art to whichthe present invention pertains without departing from the gist of thepresent invention claimed in the claims.

What is claimed is:
 1. A flexible pressure sensor comprising: a flexiblebody formed of an elastomer and in which a microchannel is formed; aconductive material formed of a conductive liquid and filling themicrochannel; and a plurality of microbumps formed of a hard materialand disposed to be in surface-contact with an upper surface or a lowersurface of a part of the microchannel.
 2. The flexible pressure sensorof claim 1, wherein the flexible pressure sensor measures a pressure byusing a change of a resistance value of the conductive material that iscaused when the microchannel filled with the conductive material isdeformed due to an external pressure, and the microbumps prevent theexternal pressure from being dispersed over the flexible body toincrease a degree of the deformation of the microchannel.
 3. Theflexible pressure sensor of claim 1, wherein the microchannel forms asingle route or multiple routes, and includes a sensing portionintegrated in a predetermined shape at a predetermined position in theflexible body, and a reservoir disposed to be spaced apart from thesensing portion, positioned at a distal end of the route, having across-sectional area larger than that of the sensing portion, and towhich a signal line for transmitting and receiving a signal to and fromthe outside is connected.
 4. The flexible pressure sensor of claim 3,wherein the microbumps are formed on the sensing portion of themicrochannel.
 5. The flexible pressure sensor of claim 3, wherein thesensing portion of the microchannel is formed in a meandering channelshape.
 6. The flexible pressure sensor of claim 3, wherein when adirection in which the microchannel extends is an extending direction, adirection parallel to a thickness of the flexible pressure sensor is athickness direction, and a direction perpendicular to the extendingdirection and the thickness direction and parallel to a width of themicrochannel is a width direction, a width or thickness of the reservoiris larger than a width or thickness of the sensing portion.
 7. Theflexible pressure sensor of claim 1, wherein when a direction in whichthe microchannel extends is an extending direction, a direction parallelto a thickness of the flexible pressure sensor is a thickness direction,and a direction perpendicular to the extending direction and thethickness direction and parallel to a width of the microchannel is awidth direction, sensitivity of the flexible pressure sensor is adjustedby adjusting a ratio (k_(t)=t/z) of a thickness (t) of the microbump toa thickness (z) between a surface of the flexible pressure sensor thatfaces the microbump, and a surface of the microchannel that faces themicrobump, or by adjusting a ratio (k_(w)=d/w) of a width (d) of themicrobump to the width (w) of the microchannel.
 8. The flexible pressuresensor of claim 7, wherein the flexible pressure sensor has an embeddedbump structure in which k_(t)<1 and k_(w)<1, an exposed bump structurein which k_(t)≥1, or an anchored bump structure in which k_(w)≥1.
 9. Theflexible pressure sensor of claim 7, wherein as k_(t) is increased, thesensitivity of the flexible pressure sensor is increased.
 10. Theflexible pressure sensor of claim 7, wherein in a case in which0<k_(w)<1, as k_(w) is increased, the sensitivity of the flexiblepressure sensor is increased, and in a case in which k_(w)≥1, as k_(w)is increased, the sensitivity of the flexible pressure sensor isdecreased.
 11. The flexible pressure sensor of claim 8, wherein themicrochannel forms a single route or multiple routes, and includes asensing portion integrated in a predetermined shape at a predeterminedposition in the flexible body, a reservoir disposed to be spaced apartfrom the sensing portion, positioned at a distal end of the route,having a cross-sectional area larger than that of the sensing portion,and to which a signal line for transmitting and receiving a signal toand from the outside is connected, and a connection portion connectingthe sensing portion and the reservoir, and the microbump having theanchored bump structure is formed in the connection portion.
 12. Amethod for manufacturing the flexible pressure sensor of claim 1, themethod comprising: a mold printing step of performing three-dimensional(3D)-printing of a micromold having a 3D shape corresponding to a shapeof the microchannel by using a sacrificial material; a bump printingstep of performing 3D-printing of the microbump at a predeterminedposition on an upper surface of the micromold by using a hard material;a primary body forming step of primarily coating a flexible materialthat is a material of the flexible body on a substrate to form aflexible material layer; a mold disposing step of disposing a coupledbody of the micromold and the microbump on an upper surface of theflexible material layer; a secondary body forming step of secondarilycoating the flexible material on the flexible material layer and anupper surface of the coupled body and hardening the flexible material toform the flexible body in which the coupled body is embedded; a channelforming step of forming the microchannel in the flexible body byremoving the micromold; and a manufacturing completion step of fillingthe microchannel with the conductive material to complete manufacturingof the flexible pressure sensor.
 13. A pulse measurement systemcomprising: the flexible pressure sensor of claim 1 that is attached toa wrist of a user and measures a pulse; and three electrodes that areattached to a body of the user, measure an electrocardiogram (ECG), andinclude Ref, In+, and In−.
 14. The pulse measurement system of claim 13,wherein the pulse measurement system calculates a blood pressure of theuser by using a pulse transit time which is a difference between a pulsepeak point and an electrocardiogram peak point.
 15. A body pressuredistribution measurement system comprising: a plurality of flexiblepressure sensors of claim 1 that are distributed on clothes of a userand perform pressure measurement; and a monitoring unit that performsreal-time monitoring of a pressure applied to a body of the user byusing a pressure measured by the plurality of flexible pressure sensors.16. The body pressure distribution measurement system of claim 15,wherein the flexible pressure sensors are distributed at positionscorresponding to bony areas including shoulders, wing bones, elbows,knees, and a tailbone which are body parts that are likely to get abedsore.